Fourier Transform Spectrometer Measures 193-nm Wavelength Standards

Using a novel Fourier transform spectrometer optimized for ultraviolet wavelengths, Gillian Nave and Craig J. Sansonetti of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., have measured the spectra of 193-nm reference standards to an uncertainty of better than five parts in 108. They hope that the work, which they reported in the February issue of the Journal of the Optical Society of America and which was funded in part by Lambda Physik, will enable semiconductor manufacturers to produce smaller feature sizes.

Nave explained that the index of refraction of the optical materials used in 193-nm exposure tools varies rapidly in the region around that wavelength. Consequently, the design of high-numerical-aperture lens systems for these tools requires that the wavelength of the illumination source be known to a high degree of precision. In practice, because sources such as Lambda Physik's NovaLine A4003 ArF excimer laser have a wavelength stability of ±0.03 pm over 100 laser pulses, it is necessary to measure wavelengths around 193 nm with an accuracy of better than one part in 107.

The company achieves this by comparing the output of the laser with particular spectral lines from materials such as iron, germanium and platinum using the optogalvanic effect, Nave said. The output of a laser is directed into a hollow-cathode lamp containing one of these materials and tuned through the region of the known spectral lines. "When the wavelength coincides with one of the lines, the voltage across the hollow cathode changes," she said. "So the spectral lines provide absolute reference wavelengths throughout the tuning curve of the laser."

But to do this, one must know with a very high accuracy what the wavelengths of those spectral lines are. In the experiments at NIST, the researchers used the FT700 vacuum-ultraviolet Fourier transform spectrometer to measure seven spectral lines of iron and platinum foils and of germanium chips in a hollow-cathode lamp. They recorded 150 to 200 scans of the spectrometer to produce each spectrum.

The novelty of the FT700, which is based on a design developed at Imperial College London and modified at NIST, is its operating range. Incorporating calcium-fluoride optics into a scanning interferometer setup, the spectrometer is optimized for the vacuum-ultraviolet and operates at wavelengths from 140 to more than 600 nm. Most Fourier transform spectrometers operate in the infrared, Nave noted, although some can operate in the visible and near-UV.

Only one other Fourier spectrometer in the world, she believes, operates in the wavelength range of the FT700.

More precise measurements of these spectral lines yield more precisely calibrated lasers and, in turn, practical benefits for semiconductor manufacturing. With accurately measured narrowband lithography lasers at their disposal, she said, designers of exposure tools can develop and incorporate lenses with higher numerical apertures than have been possible, enabling the generation of smaller feature sizes from the same illumination wavelength.